xref: /linux-6.15/kernel/rcu/srcutiny.c

// SPDX-License-Identifier: GPL-2.0+
/*
 * Sleepable Read-Copy Update mechanism for mutual exclusion,
 *	tiny version for non-preemptible single-CPU use.
 *
 * Copyright (C) IBM Corporation, 2017
 *
 * Author: Paul McKenney <paulmck@linux.ibm.com>
 */

#include <linux/export.h>
#include <linux/mutex.h>
#include <linux/preempt.h>
#include <linux/rcupdate_wait.h>
#include <linux/sched.h>
#include <linux/delay.h>
#include <linux/srcu.h>

#include <linux/rcu_node_tree.h>
#include "rcu_segcblist.h"
#include "rcu.h"

#ifndef CONFIG_TREE_RCU
int rcu_scheduler_active __read_mostly;
#else // #ifndef CONFIG_TREE_RCU
extern int rcu_scheduler_active;
#endif // #else // #ifndef CONFIG_TREE_RCU
static LIST_HEAD(srcu_boot_list);
static bool srcu_init_done;

static int init_srcu_struct_fields(struct srcu_struct *ssp)
{
	ssp->srcu_lock_nesting[0] = 0;
	ssp->srcu_lock_nesting[1] = 0;
	init_swait_queue_head(&ssp->srcu_wq);
	ssp->srcu_cb_head = NULL;
	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
	ssp->srcu_gp_running = false;
	ssp->srcu_gp_waiting = false;
	ssp->srcu_idx = 0;
	ssp->srcu_idx_max = 0;
	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
	INIT_LIST_HEAD(&ssp->srcu_work.entry);
	return 0;
}

#ifdef CONFIG_DEBUG_LOCK_ALLOC

int __init_srcu_struct(struct srcu_struct *ssp, const char *name,
		       struct lock_class_key *key)
{
	/* Don't re-initialize a lock while it is held. */
	debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
	lockdep_init_map(&ssp->dep_map, name, key, 0);
	return init_srcu_struct_fields(ssp);
}
EXPORT_SYMBOL_GPL(__init_srcu_struct);

#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/*
 * init_srcu_struct - initialize a sleep-RCU structure
 * @ssp: structure to initialize.
 *
 * Must invoke this on a given srcu_struct before passing that srcu_struct
 * to any other function.  Each srcu_struct represents a separate domain
 * of SRCU protection.
 */
int init_srcu_struct(struct srcu_struct *ssp)
{
	return init_srcu_struct_fields(ssp);
}
EXPORT_SYMBOL_GPL(init_srcu_struct);

#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/*
 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
 * @ssp: structure to clean up.
 *
 * Must invoke this after you are finished using a given srcu_struct that
 * was initialized via init_srcu_struct(), else you leak memory.
 */
void cleanup_srcu_struct(struct srcu_struct *ssp)
{
	WARN_ON(ssp->srcu_lock_nesting[0] || ssp->srcu_lock_nesting[1]);
	flush_work(&ssp->srcu_work);
	WARN_ON(ssp->srcu_gp_running);
	WARN_ON(ssp->srcu_gp_waiting);
	WARN_ON(ssp->srcu_cb_head);
	WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
	WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
	WARN_ON(ssp->srcu_idx & 0x1);
}
EXPORT_SYMBOL_GPL(cleanup_srcu_struct);

/*
 * Removes the count for the old reader from the appropriate element of
 * the srcu_struct.
 */
void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
{
	int newval;

	preempt_disable();  // Needed for PREEMPT_LAZY
	newval = READ_ONCE(ssp->srcu_lock_nesting[idx]) - 1;
	WRITE_ONCE(ssp->srcu_lock_nesting[idx], newval);
	preempt_enable();
	if (!newval && READ_ONCE(ssp->srcu_gp_waiting) && in_task())
		swake_up_one(&ssp->srcu_wq);
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock);

/*
 * Workqueue handler to drive one grace period and invoke any callbacks
 * that become ready as a result.  Single-CPU and !PREEMPTION operation
 * means that we get away with murder on synchronization.  ;-)
 */
void srcu_drive_gp(struct work_struct *wp)
{
	int idx;
	struct rcu_head *lh;
	struct rcu_head *rhp;
	struct srcu_struct *ssp;

	ssp = container_of(wp, struct srcu_struct, srcu_work);
	preempt_disable();  // Needed for PREEMPT_LAZY
	if (ssp->srcu_gp_running || ULONG_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) {
		preempt_enable();
		return; /* Already running or nothing to do. */
	}

	/* Remove recently arrived callbacks and wait for readers. */
	WRITE_ONCE(ssp->srcu_gp_running, true);
	local_irq_disable();
	lh = ssp->srcu_cb_head;
	ssp->srcu_cb_head = NULL;
	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
	local_irq_enable();
	idx = (ssp->srcu_idx & 0x2) / 2;
	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
	preempt_enable();
	swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
	preempt_disable();  // Needed for PREEMPT_LAZY
	WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
	preempt_enable();

	/* Invoke the callbacks we removed above. */
	while (lh) {
		rhp = lh;
		lh = lh->next;
		debug_rcu_head_callback(rhp);
		local_bh_disable();
		rhp->func(rhp);
		local_bh_enable();
	}

	/*
	 * Enable rescheduling, and if there are more callbacks,
	 * reschedule ourselves.  This can race with a call_srcu()
	 * at interrupt level, but the ->srcu_gp_running checks will
	 * straighten that out.
	 */
	preempt_disable();  // Needed for PREEMPT_LAZY
	WRITE_ONCE(ssp->srcu_gp_running, false);
	idx = ULONG_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max));
	preempt_enable();
	if (idx)
		schedule_work(&ssp->srcu_work);
}
EXPORT_SYMBOL_GPL(srcu_drive_gp);

static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
	unsigned long cookie;

	preempt_disable();  // Needed for PREEMPT_LAZY
	cookie = get_state_synchronize_srcu(ssp);
	if (ULONG_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) {
		preempt_enable();
		return;
	}
	WRITE_ONCE(ssp->srcu_idx_max, cookie);
	if (!READ_ONCE(ssp->srcu_gp_running)) {
		if (likely(srcu_init_done))
			schedule_work(&ssp->srcu_work);
		else if (list_empty(&ssp->srcu_work.entry))
			list_add(&ssp->srcu_work.entry, &srcu_boot_list);
	}
	preempt_enable();
}

/*
 * Enqueue an SRCU callback on the specified srcu_struct structure,
 * initiating grace-period processing if it is not already running.
 */
void call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
	       rcu_callback_t func)
{
	unsigned long flags;

	rhp->func = func;
	rhp->next = NULL;
	preempt_disable();  // Needed for PREEMPT_LAZY
	local_irq_save(flags);
	*ssp->srcu_cb_tail = rhp;
	ssp->srcu_cb_tail = &rhp->next;
	local_irq_restore(flags);
	srcu_gp_start_if_needed(ssp);
	preempt_enable();
}
EXPORT_SYMBOL_GPL(call_srcu);

/*
 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
 */
void synchronize_srcu(struct srcu_struct *ssp)
{
	struct rcu_synchronize rs;

	srcu_lock_sync(&ssp->dep_map);

	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
			lock_is_held(&rcu_bh_lock_map) ||
			lock_is_held(&rcu_lock_map) ||
			lock_is_held(&rcu_sched_lock_map),
			"Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");

	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
		return;

	might_sleep();
	init_rcu_head_on_stack(&rs.head);
	init_completion(&rs.completion);
	call_srcu(ssp, &rs.head, wakeme_after_rcu);
	wait_for_completion(&rs.completion);
	destroy_rcu_head_on_stack(&rs.head);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);

/*
 * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
 */
unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
{
	unsigned long ret;

	barrier();
	ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
	barrier();
	return ret;
}
EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);

/*
 * start_poll_synchronize_srcu - Provide cookie and start grace period
 *
 * The difference between this and get_state_synchronize_srcu() is that
 * this function ensures that the poll_state_synchronize_srcu() will
 * eventually return the value true.
 */
unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
{
	unsigned long ret;

	preempt_disable();  // Needed for PREEMPT_LAZY
	ret = get_state_synchronize_srcu(ssp);
	srcu_gp_start_if_needed(ssp);
	preempt_enable();
	return ret;
}
EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);

/*
 * poll_state_synchronize_srcu - Has cookie's grace period ended?
 */
bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
{
	unsigned long cur_s = READ_ONCE(ssp->srcu_idx);

	barrier();
	return cookie == SRCU_GET_STATE_COMPLETED ||
	       ULONG_CMP_GE(cur_s, cookie) || ULONG_CMP_LT(cur_s, cookie - 3);
}
EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);

#ifndef CONFIG_TREE_RCU
/* Lockdep diagnostics.  */
void __init rcu_scheduler_starting(void)
{
	rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
}
#endif // #ifndef CONFIG_TREE_RCU

/*
 * Queue work for srcu_struct structures with early boot callbacks.
 * The work won't actually execute until the workqueue initialization
 * phase that takes place after the scheduler starts.
 */
void __init srcu_init(void)
{
	struct srcu_struct *ssp;

	srcu_init_done = true;
	while (!list_empty(&srcu_boot_list)) {
		ssp = list_first_entry(&srcu_boot_list,
				      struct srcu_struct, srcu_work.entry);
		list_del_init(&ssp->srcu_work.entry);
		schedule_work(&ssp->srcu_work);
	}
}